The invention relates to a spray drier for the preparation of powders, agglomerates and the like, by the drying of pumpable products.
Spray driers of this type are known in different configurations as disk, nozzle and fluidizing spray driers and are also used as agglomerators. Different operating modes and configurations of such spray driers are described in Chem.-Ing. Tech. 59 (1987), No. 2, pages 112 to 117. As a brief summary, three principal variants of spray driers are described; i.e., disk spray driers with a swirling distribution of the hot air serving as the drying gas, nozzle spray driers with or without a swirling distribution of the drying gas and finally, fluidized spray driers with an axial hot air supply from the top, a fluid bed on the bottom and powder recycling into the drying chamber, which is in the form of a tower. As a rule the wet material to be dried and sprayed is introduced from the top. The drying chambers have two principle outlets, i.e. the product discharge for discharging the coarse, grain material or product material proportion directly from the drying chamber or laterally from the integrated fluid bed and the discharge line for the drying gas or exhaust gas, which carries with it a fine proportion of the product. The fine material proportion entrained by the drying gas or the exhaust gas is recovered by cyclones and/or filters and may either be recycled, (primarily in installations which are able to carry out agglomerations) into the drying chamber, or may be further processed separately.
The quantity of this fine material component which is entrained by the drying gas depends on different factors, such as the fineness of the atomization, the proportion of the product itself, the agglomeration effect, and the precipitation within the drying chamber.
Usually, the product is demanded in a form in which it contains only very small quantities of the fine material, so as not to exceed the minimum required grain sizes, and in particular to prevent excessive dusting. It can be very expensive to re-process the fine material components obtained, so that they again are usable.
Different layouts are known whereby the coarse material yield is improved and the excessive entrainment of coarse material in the exhaust air prevented, and wherein it is also intended to insure that the smallest possible quantity of fine material arrives in the coarse material discharge of the tower or drying chamber.
Thus, for example, an embodiment is known from U.S. Pat. No. 2,280,073, wherein the drying air is blown in by a cyclone. The cyclonic flow of the drying air in combination with the rotating atomization of the wet material is intended to effect the separation of the dry coarse material from the drying gas as a result of the higher centrifugal force acting on the material. The rotating atomizer located in front of the coaxial drying air inlet and outlet openings also acts as a fan, thereby enhancing the suction effect of an external pump. To separate the fine material extrained in the drying air, expensive filter installations or the like are provided in the exhaust flow direction downstream of the fan. The apparatus described is designed for a specific product and is relatively difficult to reset for products with different grain size distributions.
Spray driers are also known in which the exhaust air is discharged centrally by means of a kind of snorkel projecting radially into the lower area of the drying chamber, or installed axially on the bottom. The separation of the coarse material proportion from the proportion of fine material depends primarily on the swirl intensity of the drying gas within the drying chamber and the flow conditions around and upon its entry into the discharge line. Configurations are known for example from U.S. Pat. No. 3,963,559, British Patent Publication GB 2 020 986 or German Patent Publication DE 849,350, wherein in order to obtain suitable flow conditions, additional covers in the form of hats or the like, are placed over the discharge line. The separating apparatus known from CH 374 972 obtains this effect by a hat-shaped rotor in combination with a stator. Other configurations provide a plurality of tangential nozzles in the area of the discharge line in order to create conditions of flow, whereby the dust particles dispersed into the air are divided into coarse and fine components.
Spray driers, such as that described for example in U.S. Pat. No. 1,426,030, are further known, in which cyclones are used for the separation of the coarse material from the drying medium, and wherein the fine material entrained in the drying air is again recycled into the drying chamber. Additional spray driers of this type, used mainly for agglomeration, contain two outlets for the exhaust air in the roof of the drying chamber or tower. In these installations the separation of the coarse and fine material depends primarily on the mean axial flow velocity within the drying chamber, under the roof.
A common problem with all of these spray driers is that too much coarse material (grains) is being exhausted with the fine material. Although this may be recovered by subsequent screening or sifting, to do so would require additional space, together with filters or sifters. Furthermore, there is the risk that the product may be damaged for example in a cyclone, so that the grains would become fine. Finally, screening has a relatively low efficiency. A solution which uses tangential nozzles within the cone of the tower involves the risk of damage to the product in the outlet cone by abrasion, and requires the exhaust air filter to be dimensioned larger in view of the recycled volume of air. In a further solution which uses a moving hat, the moving hat requires a relatively large amount of space within the drying chamber and reduces the drop height which is important for drying. Furthermore, the product impacts upon the moving hat and becomes encrusted at the upper tip of the moving hat.
All of the known apparatuses have relatively large dimensions for the hat diameter or the tower diameter and thus need more space and have correspondingly higher costs. They are for the most part designed in a product specific manner and may be, if at all, only reset for other grain size distributions with difficulty.